Bartonellosis, caused by infection with the bacteria Bartonella henselae or B. quintana, is emerging as an important opportunistic infection in patients with AIDS. Bartonella henselae is the etiologic agent of cat-scratch disease, while B. quintana is the agent of "trench fever", both self-limiting infections in immunocompetent hosts. Bartonella infection in AIDS patients, or other immune compromised hosts, however, has been associated with a severe life threatening proliferation of vascular tissue affecting multiple organ systems; a condition referred to as Bacillary Angiomatosis (BA). The pathogenetic mechanisms leading to the development of BA are unknown. We are utilizing the SIV/macaque model system to determine the pathogenesis of Bartonella infection in SIV-infected, immunocompromised macaques, and to evaluate the utility of the SIV/macaque system as a potential model for BA. SIV-infected animals, at various stages of immunodeficiency, will be inoculated intradermally with B. henselae or B. quintana organisms, and monitored for onset, duration, immune response, and clinical outcome of bacteremia. In preliminary data obtained from one SIV-infected macaque (CD4+ lymphocyte count = 400-500/ul) inoculated with B. quintana, prolonged bacteremia (> 120 days) has been documented, associated with a strong antibody response. However, no clinical signs or systemic affects associated with the bacteremia have been observed, other than a transient fever and skin lesions at the site of inoculation 1-2 weeks post-bartonella infection. BA may be a very late manifestation of chronic bartonellosis, and long-term follow-up of these animals is planned. Additional inoculations with B. quintana as well as B. henselae will be performed using SIV-infected macaques with more advanced immunosuppression. The SIV/macaque model system provides an opportunity to study the pathogenesis of BA, and other AIDS-associated opportunistic pathogens. *KEY*Bartonella, Bacillary angiomatosis, SIV, AIDS, Macaque